If it is attempted to obtain vehicle-mounted audio equipment with higher power and higher quality, the power amplifier therein tends to become larger in size. Therefore, a power amplifier independent of the head unit is commonly used.
The input to the power amplifier can be either a digital signal or an analog signal. Since a head unit includes analog audio sources such as a radio other than digital audio sources such as a CD, analog signals are generally outputted from the head unit.
Since digital signal processors for audio signal processing have come to be used recently, those including an A/D converter and a D/A converter have vehicle-mounted audio equipment. By the employment of such audio signal processing DSPs, it has become possible to realize filters and the like having desired characteristics in power amplifiers with ease and at low cost.
While such an audio signal processing DSP is used in a power amplifier, if an analog signal with a level higher than a full-scale level is inputted to an A/D converter at the pre-stage of the audio signal processing DSP, a digital clip occurs to produce a very discordant noise. Therefore, the analog audio signal is required to be suppressed at a level sufficiently lower than the full-scale level. Suppressing the analog audio signal level to such a level degrades the signal-to-noise ratio (SNR), and therefore, the number of bits processed in the digital processing at the post stage is substantially reduced so that the quality of sound is deteriorated.
Accordingly, instead of greatly lowering the analog audio signal level, there is a way whereby the deterioration in the quality of sound can be prevented by having an overflow from an A/D converter detected and the signal is compensated so as not to produce a digital clip.
Since a digital clip produces a larger distortion than an analog clip dependent on the power supply voltage, it causes great annoyance to the ear. Hence, the overflow detection and the data correction are required to be made at high speed.
As a conventional digital signal processing amplifier, there is one shown in FIG. 5.
FIG. 5 is a schematic diagram showing a configuration of the conventional digital signal processing amplifier. In FIG. 5, the conventional digital signal processing amplifier includes input buffer amplifiers 1a–1c made up of a plurality of amplifiers with different gains, A/D converter 2 for converting an analog output signal from input buffer 1a, 1b or 1c into a digital signal, switches 3a–3c inserted between input buffers 1a–1c and A/D converter 2 for making and breaking connections therebetween, and gain controller 4 for monitoring an overflow of the output digital signal from A/D converter 2 to thereby control closing and opening of switches 3a–3c. 
Operation of the conventional digital signal processing amplifier will be described below.
An analog input signal from the input terminal is inputted to input buffer amplifiers 1a–1c with different gains and analog output signals at different signal levels are outputted from input buffer amplifiers 1a–1c, respectively. Suppose now that switch 3a between input buffer amplifier 1a with the smallest gain and A/D converter 2 is closed and the other switches 3b, 3c are open. Then, an analog output signal from input buffer amplifier 1a at the lowest signal level is inputted to A/D converter 2 and the input signal is converted into a digital signal and outputted to the output terminal. Meanwhile, this digital output signal is also inputted to gain controller 4, and when there is a sufficient margin of the signal level to be processed, the switch is turned over from switch 1a to switch 1b or 1c, so that the range to the full-scale of A/D converter 2 is used to the extent possible.
In other words, gain controller 4 selects one out of input buffer amplifiers 1a–1c so that the output value of A/D converter 2 at the current time may be given an optimum dynamic range corresponding to its full-scale level. When the input signal level to A/D converter is reduced, its dynamic range is decreased. In other words, the number of bits used for the digital signal is decreased. Therefore, when the signal is that for an audio signal, the quality of sound becomes deteriorated and, hence, it is desired that the input signal level to A/D converter 2 be held as high as possible.
On the other hand, when an input signal level to A/D converter 2 reaches as high a level as its full-scale level, the signal becomes unable to be properly converted as a digital signal. In the case where the signal is an audio signal, a large distortion known as a digital clip is produced. Hence, it becomes necessary to select one amplifier out of input buffer amplifiers 3a–3c with the use of switches 3a–3c to prevent the occurrence of such a digital clip.
As prior art references related to the invention of this application, there are known, for example, Japanese Patent Unexamined Publication No. H07-273650, Japanese Patent Unexamined Publication No. H07-44999, and Japanese Utility Model Unexamined Publication No H04-132749.
However, such a problem is involved in digital signal processing amplifiers of the prior art examples that dynamically switching the gain in accordance with the signal level, especially with an audio signal or the like, is difficult to attain.